This invention is in the field of methods for forming and deep drawing thermoplastic sheet and particularly for forming perforated thermoplatic canvas as used in needlepoint work.
Popular needlepoint canvas is a mesh sheet of about 1/16 of an inch thick with a matrix of perpendicular threads defining therebetween a vast plurality of essentially square apertures. For many years these needlepoint canvases have been used in their original flat state, so that the resulting product is a stitched sheet which remains essentially flat like a placemat for a table or a wall-hanging.
To increase the possibilities for interesting and useful shapes of needlepoint canvas, attempts have been made to form the plastic canvases into different shapes, particularly curved shapes. These efforts have been less than satisfactory, and especially where deep drawing has been attempted the sheets have either ruptured in the area of the deep drawn curve or become too thin to use.
Various techniques have been attempted to overcome the problem of deep drawing thermoplastic materials, including roughening the end of a punching die or plunger so that the end part which contacts the sheet to be drawn engages said sheet with increased friction. This tends to protect the engaged part from excessive stretching and bending while inducing other parts of the sheet remote from the end of the punch die to stretch more uniformly. Other attempts to solve the problem include varying the temperature of the thermoplastic sheet, and particularly heating it to a softer state, however this usually led to greater rupturing rather than less.
The needlepoint canvases comprising essentially threads formed into a matrix comprise a fabric which is particularly fragile and susceptible of rupturing in the deep drawing processes regardless of techniques described above. The new invention seeks to provide a feasible economic process and apparatus for forming the peculiar thermoplastic mesh canvases as used in needlepoint into curved shapes, particularly a hemispherical curve or other deep drawn shapes. In seeking an economic technique, one object is to provide a process which can operate at room temperature and thus avoid heating costs. A further object is for the procedure to be very quick and simple and thus to be adaptable for mass production. These objectives have been achieved as described in the summary of the invention below and in the detailed description of the preferred embodiment thereafter.